Simplify Configure in a few ways

- No need to distinguish between gcc-llvm and clang. First of all,
gcc-llvm is quite old and surely unmaintained by now. Second of all,
none of the code actually care about that distinction!
Now, it does make sense to consider C multiple frontends for LLVMs in
the form of clang vs clang-cl (same clang, yes, but tweaked
interface). But this is better handled in terms of "gccish vs
mvscish" and "is LLVM", yielding 4 combinations. Therefore, I don't
think it is useful saving the existing code for that.
- Get the remaining CC_LLVM_BACKEND, and also TABLES_NEXT_TO_CODE in
mk/config.h the normal way, rather than hacking it post-hoc. No point
keeping these special cases around for now reason.
- Get rid of hand-rolled `die` function and just use `AC_MSG_ERROR`.
- Abstract check + flag override for unregisterised and tables next to
code.
Oh, and as part of the above I also renamed/combined some variables
where it felt appropriate.
- GccIsClang -> CcLlvmBackend. This is for `AC_SUBST`, like the other
Camal case ones. It was never about gcc-llvm, or Apple's renamed clang,
to be clear.
- llvm_CC_FLAVOR -> CC_LLVM_BACKEND. This is for `AC_DEFINE`, like the
other all-caps snake case ones. llvm_CC_FLAVOR was just silly
indirection *and* an odd name to boot.